LUBRICATION SCIENCE Lubrication Science 2009; 21: 169–182 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/ls.85

Tribological performance of ionised vegetable oils as lubricity and fatty oiliness additives in lubricants and fuels Michel Roegiers and Boris Zhmud*,† E-ION s.a., 18, Val des Seigneurs, B-1150 Brussels, Belgium‡

ABSTRACT Lubricity and fatty oiliness additives, also known as friction modifiers in the tribological vocabulary, are steadily gaining acceptance from lubrication engineers and lubricant formulators. The present communication describes how such additives function in various tribosystems and which parameters control lubricity of finished formulations. Extensive experimental data are presented to demonstrate the outstanding tribological performance of biobased lubricity and fatty oiliness additives produced by ElektrionizationTM of vegetable feedstocks. Featuring a unique combination of viscosity and polarity, ionised vegetable oils form sufficiently thick and resilient protective layers by adsorption to rubbing surfaces. It is shown that, unlike extreme pressure additives, which act when a direct asperity–asperity contact occurs in the boundary lubrication regime, ionised vegetable oils function by postponing the onset of the boundary lubrication regime. Copyright © 2009 John Wiley & Sons, Ltd. key words: lubricity

ionised vegetable oil; friction modifier; lubricity additive; fuel-economy engine oil; diesel fuel

INTRODUCTION The terms ‘lubricity’ and ‘oiliness’ refer to slipperiness of lubricant films formed in boundary lubrication, a condition which lies between unlubricated sliding and fluid–film lubrication, and which is also defined as that condition of lubrication in which the friction between the surface is determined by the properties of the surfaces and properties of the lubricant other than viscosity. Boundary lubrication encompasses a significant portion of lubrication phenomena and commonly occurs during the starting and stopping of machines. The function of lubricants in tribological systems is to reduce friction and wear. The reduction of friction results from the formation of a lubricant film separating the rubbing surfaces. As a rule of thumb, lubricant films are anisotropic and reveal fairly complex non-Newtonian rheology. As a result, the coefficient of friction is not constant — it depends on the applied load, film deformation (elastic effects), sliding speed (viscous effects), as well as on their time derivatives. *Correspondence to: Boris Zhmud, E-ION s.a., 18, Val des Seigneurs, B-1150 Brussels, Belgium. † E-mail: [email protected] ‡ www.eion-additives.com

Copyright © 2009 John Wiley & Sons, Ltd.

170

M. ROEGIERS AND B. ZHMUD

The thickness of the lubricant film depends upon constituent chemistry (base oil and additives), as well as upon the operating conditions, specifically the applied load and the sliding velocity. At a sufficiently high load or low sliding speed — the condition encountered e.g. for connecting rod journals in internal combustion engines near the top dead centre — lubricant may be expelled from the friction zone, leaving the rubbing surface unlubricated. In this case, severe friction and intense wear result. Previously, in the field of fuels and lubricants, lubricity had always been taken for granted. However, the situation has started to change in the past two decades. Lubricity — or rather the lack thereof — has become a ‘hot topic’ in the beginning of the 1990s, following the introduction of ultra-low sulphur diesel (ULSD).1,2 The major benefit of the change to ULSD is that the environmental impact of emissions of sulphur dioxide is greatly reduced. However, soon it has been realised that a reduction in the sulphur content also causes a reduction in the fuel lubricity, blamed for premature fuel pump and injector wear. It was this new challenge that spurred American Society for Testing and Materials (ASTM) to include lubricity into the standard specification D975 for diesel fuel oils. At the same time, in the lubricant branch, ever-growing quality demands and stringent environmental regulations have led to broad commercialisation of hydrocracking, catalytic dewaxing and hydrofinishing technologies creating an abundant supply of API Group II and III base oils. The soaring crude oil price also drives development of other competing technologies, such as gas-to-liquid (GTL) conversion using the Fischer–Tropsch process. However, despite many undisputed advantages over their Group I predecessors, new base oils produced using the all-hydrogen route or the GTL conversion have one major drawback — they lack solvency and lubricity. To alleviate the dramatic effect of ‘dry’ friction, extreme pressure (EP) additives are deployed. Those additives — normally containing sulphur, phosphorus or chlorine — are capable of reacting with the material of rubbing surfaces to form a thin surface layer of sulphide, phosphate or chloride, which acts as a solid lubricant when the rubbing surfaces come into a direct contact with each other. When unlubricated sliding is encountered, friction and material deformation generate enough heat to trigger the EP reactions. However, the price of that is a microscopic scar left at the surface. Such scars are accumulated with time to give wear. It is important to realise that EP additives start to act after the asperity–asperity contact has occurred, but they do not prevent its occurrence. The latter task is left to friction modifiers, compounds that form soft but sufficiently resilient protective layers by adsorption to rubbing surfaces.3 Depending on the molecular structure and chemistry of friction modifier, a brush-like dense monolayer or a spongelike diffuse multilayer will form; the latter is capable of retaining base lubricant in the friction zone. When the protective layer is mechanically deformed under the applied load, a disjoining pressure builds up, pushing the rubbing surfaces apart. A number of theories exist attempting to explain the mechanism of short-range sterical repulsion between surfaces containing adsorbed layers of polar components.4 Most readers of this article have probably experienced such a lubricity-enhancing effect while walking on the slippery rocks of the seashore. The algae slime growing on the rocks retains a sufficiently thick layer of water, which acts as a lubricant between your feet and the rock surface. The mechanism of action of such a friction modifier is explained in Figure 1. The formation of such a surface film has a beneficial tribological effect as it reduces friction and wear, as well as associated energy dissipation, noise generation and tribomutation. It is interesting to note that a lubricityenhancing effect may also be achieved by surface texturing, whereby a system of surface pores is generated to keep lubricant in friction points.5 Copyright © 2009 John Wiley & Sons, Ltd.

Lubrication Science 2009; 21: 169–182 DOI: 10.1002/ls

TRIBOLOGICAL PERFORMANCE OF IONISED VEGETABLE OILS

171

stress delocalization and wear reduction

surface 1 EP layer FM layer lubricant oil FM layer EP layer

worn material and sludge solubilization

surface 2

Figure 1. Mechanism of friction- and wear-reducing action of friction modifiers forming a protective surface layer by adsorption.

A good friction modifier is expected to combine a sufficiently high adsorptivity to metal surfaces (which is largely controlled by its polarity) and the ability to form a sufficiently thick and resilient protective film (which is basically determined by its viscosity), as well as the ability to dissolve in base oils where it is meant to be used in. These are antagonistic properties, as increasing polarity reduces miscibility with mineral base oils. For instance, polyethylene glycols are highly polar but immiscible with mineral oils. Heavy polyalphaolefins, on the contrary, are nonpolar at all; as a result, they have no surface activity whatsoever. Many synthetic esters also lack polarity and may desorb with increasing oil temperature, leaving the surface unprotected. Thickness of the adsorbed layer normally increases with increasing the viscosity, which is linked to increased average molecular weight and intermolecular cohesion. Molecular structure has significance, too — most friction modifiers have amphiphilic molecules similar to those of surfactants. Other important factors are thermal stability, oxidation stability, as well as ecological and health safety considerations. The majority of base oils are the so-called normal fluids demonstrating nearly Newtonian rheological behaviour at low sliding velocities. In this case, even moderate loads may displace the oil from the friction contact zone. For instance, in a car engine, the cylinder liner–piston ring tribosystem has two dead points in which load is combined with zero sliding velocity. At those points, thinner fueleconomy engine oils — such as those of 0W-20 specification — may fail to provide adequate film thickness unless appropriate film-thickening lubricity components are included in their formulations.6–9 Unlike extreme pressure additives, which act when a direct surface-to-surface contact occurs in the boundary lubrication regime, ionised vegetable oils function by postponing the onset of the boundary lubrication regime as explained in Figure 2. Equivalently, one may talk about expanding the borders of the hydrodynamic lubrication regime: in a loading cycle (when moving from the right to the left over the Stribeck curve), the film lubrication will stay longer and stand higher loads, and in an unloading cycle (moving from the left to the right), the change from boundary lubrication to hydrodynamic lubrication will occur earlier. Ionised vegetable oils can also be regarded as mild anti-wear agents working at moderate temperatures and loads where conventional anti-wear and EP additives, such as tricresylphosphate, zinc dialkyldithiophosphate and methylene-bis(di-n-butyldithiocarbamate), to mention a few, are not yet reactive. Copyright © 2009 John Wiley & Sons, Ltd.

Lubrication Science 2009; 21: 169–182 DOI: 10.1002/ls

172

M. ROEGIERS AND B. ZHMUD

no additives FM additive EP additive FM and EP

log µ

EP effect synergetic effect of FM and EP additives

FM effect

boundary lubrication

film lubrication

log (ηv/p) Figure 2. Comparison of the tribological effects produced by extreme pressure (EP) additives and by friction modifiers (FM) such as ionised vegetable oils. The logarithm of the coefficient of friction, µ, is plotted against the logarithm of the Hersey number.

The protective layers formed by such Fiction Modifiers are self-regenerating: if the layer is damaged by applying too high a stress, it will be restored by adsorption of a new portion of friction modifier from the bulk and by lateral diffusion of adsorbed molecules caused by a surface pressure gradient. GOALS OF THE PRESENT RESEARCH The primary goal of this communication is to demonstrate the outstanding tribological performance of bio-based lubricity and fatty oiliness additives produced by ElektrionizationTM of vegetable feedstocks, a proprietary process whereby feedstocks go through electro-ionising treatment.10,11 This leads to an increase in viscosity, polarity and viscosity index, due probably to partial polymerisation and branching of fatty chains and the formation of small amounts of free fatty acids and mono- and biglycerides.12 No catalyst is used, nor any synthetic additives. Increasing the polar functionality of molecules of vegetable oil has a positive impact on friction and wear protection resulting from stronger adsorption on metal surface as well as from the thermodynamic stabilisation of the adsorbed layers by cross linking and lateral interaction between adsorbed molecules.13 In this paper, a particular type of ionised vegetable oil available commercially under E-ION R (formerly, Elektrion R) trade name is studied. The physicochemical properties of this additive are described in the Experimental section. EXPERIMENTAL Tribological tests were carried out using an in-house tribotester (steel vs steel) specially designed to enable accurate friction coefficient measurements in hydrodynamic, elastohydrodynamic and boundary lubrication regimes. The conceptual instrumental setup is outlined in Figure 3 and consists of two steel Copyright © 2009 John Wiley & Sons, Ltd.

Lubrication Science 2009; 21: 169–182 DOI: 10.1002/ls

TRIBOLOGICAL PERFORMANCE OF IONISED VEGETABLE OILS

173

Figure 3. Scheme of the tribometer used for Stribeck curve measurement: (1) mounting plate; (2) container filled with oil; (3) rotating steel disk; (4) steel pads pressed against the disk; (5) rods for applying pressure to the pads; (6) flex-spring for controlling torque; (7) laser-based optical sensor for measuring angular deviation.

pads pressed against a rotating steel disk immersed in the lubricants in study. The rotation speed and the pressure applied to the pads can be varied, and the resulting torque produced by the friction force measured, whereby the coefficient of friction is derived as a function of the Hersey number (viscosity × sliding velocity / applied pressure). Testing results were presented in the form of Stribeck diagrams plotting the coefficient of friction versus the Hersey number.14 The Hersey number may be considered as a measure of effective lubricant film thickness between the rubbing surfaces. The use of Stribeck plots allows a compact and consistent representation of data collected at various loads and sliding velocities. The actual pressure settings were 1.96, 3.92, 5.89, 7.85 and 9.81 bar (1 bar = 100 kPa). The test conditions were adjusted so as to cover the transition from the elasto-hydrodynamic (EHD) to the boundary lubrication regime. Essential physicochemical characteristics of lubricants and fuels used in the tests are summarised in Tables I–V.

RESULTS AND DISCUSSION Tests on the Lubricity of the Jet Fuel In the early 1990s, lubricity has become a concern for fuels after the introduction of ULSD.1,2 The first field experience with ULSD in Sweden was quite a disaster, as numerous failures of fuel pumps and increased injector wear were reported and later linked to the fact that the removal of sulphur (